Since carbon fibers have high specific strength and specific modulus of elasticity, they are used in a variety of resin materices in the form of long or short filaments to form fiber-reinforced composites. In aerospace applications that require high mechanical properties and heat resistance, and in other general industrial fields, thermosetting resins such as unsaturated polyester resins, epoxy resins and polyimide resins have been used as matrix resins. However, since these matrix resins are brittle and inferior in impact resistance, improvement thereof has been required, especially for use in aircraft.
Other problems with the conventional thermosetting resins are that prepregs made from them are difficult to store in good condition because of their short shelf life and that such prepregs lend themselves to have low production rate since they require a long time to be formed into a desired shape.
As compared with prepregs of thermosetting resins, those of thermoplastic resins have distinct advantages in that they can be formed into composites having improved impact resistance, that they are fairly easy to store in good conditions, and that they can be shaped into a desired form in a short period of time, thereby contributing to a lower shaping cost.
Commonly employed methods for producing prepregs of thermoplastic resins include: (1) hot-melt-adhering a film of thermoplastic resins to carbon fibers (described e.g., in Japanese Patent Application (OPI) No. 29651/83); (2) immersing a carbon fiber strand in an aqueous dispersion containing thermoplastic resin, particles, melting the resin particles and drying the carbon fiber strand (described e.g.,in Brit. Patent 1,424,168); and (3) passing carbon fibers through a fluidized bed of a resin powder to have the resin deposited on the fibers and heating the resin to melt (described in e.g., U.S. Patent 3,742,106). However, these methods each has problems. In the first method since the resin does not permeate into the inside of the carbon fiber strand sufficiently their mechanical characteristics can not be obtained sufficiently. The second method has the disadvantage that spreadability of a carbon fiber is poor, which results in low impregnation of the resin into the carbon fiber strand. In the third method, it is difficult to control the amount of resin particles to be deposited on the carbon fibers.
The present inventors conducted studies in order to develop a process for producing a product of carbon fiber reinforced thermoplastic resin without suffering from the aforementioned problems of the prior art techniques.